This invention relates to gradient magnetic field coils used for a nuclear magnetic resonance imaging apparatus (hereinafter referred to as the "MRI imaging apparatus") and a bobbin for holding the gradient magnetic field coils. More specifically, the present invention relates to gradient magnetic field coils which allow easy setting of a patient to the MRI apparatus, and can be fitted and removed easily to and from the MRI imaging apparatus, and the structure of a bobbin of the gradient magnetic field coil.
A technology of obtaining a tomographic image of an inspection object such as proton density imaging by utilizing nuclear magnetic resonance has been developed in the past. To speed up the imaging operation of the tomographic image or to obtain higher resolution according to this prior art technology, it has been a customary practice to reduce the distance between the gradient magnetic field coils and the patient and to apply a larage current of at most hundreds of amperes to the gradient magnetic fields because strong gradient magnetic fields must be applied to the object of imaging.
When a current flows through a gradient magnetic field coil, the gradient magnetic field coil receives a Lorentz force from a static magnetic field and undergoes vibration. Since a bobbin for holding the gradient magnetic field coil holds the coil in such a manner as to suppress this vibration, it is strongly produced and its weight tends to increase, so that mounting and removal of the bobbin to and from the MRI imaging apparatus has become more difficult. For this reason, the gradient magnetic field coil produced under a predetermined condition is held by the bobbin add under such a state, is fixedly mounted to the MRI imaging apparatus in many cases.
The prior art technologies related with the present inventioin are as follows. First, a method of reducing the distance between the gradient magnetic field coil and the patient by dividedly using a bobbin for holding the gradient magnetic field coil for each imaging portion of the subject while keeping the size of the bobbin unaltered is described in (a) JP-A-2-1238 and (b) JP-U-3-36606. The method of (a) JP-A-2-1238 sets at least one gradient magnetic field coil closer to the body of the patient, applies a predetermined current to a gradient magnetic field coil set and generates a stronger gradient magnetic field inside the body of the patient. The method of (b) JP-U-3-36606 mounts the gradient magnetic field coil to an air bag, introduces or evacuates compressed air into and from this air bag and thus fits and removes the gradient magnetic field coil to and from the MRI imaging apparatus.
A method which makes the size and shape of the radio-frequency magnetic field coil changeable in accordance with the physical features of the patient is described in (c) JP-A-59-32855 and (d) JP-A-63-501336 (PCT). The bobbin of the radio-frequency magnetic field coil is smaller in size and lighter in weight than the bobbin of the gradient magnetic field coil, and can be fitted and removed more easily. For the field intensity generated by the radio-frequency magnetic field is from several .mu.T to dozens of .mu.T which is about 1/10,000 of the field intensity generated by the gradient magnetic field during high speed imaging, and a large current need not be caused to flow through the radio-frequency magnetic field coil.